The presence of large deposits of oil shale in the semi-arid high plateau region of the western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,595; 4,043,596; 4,043,597; and 4,043,598 which are incorporated herein by this reference. These patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale as described in U.S. Pat. No. 3,661,423 includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products and a residual solid carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off-gas is also withdrawn from the bottom of the retort. Such off-gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process. The products of retorting are referred to herein as liquid and gaseous products.
There are several reasons why it is desirable to know the temperature of the combustion zone as it advances through an in situ oil shale retort.
It is desired to maintain the combustion zone temperature below about 2100.degree. F. because the oil shale fuses, thereby plugging the retort when heated above this temperature.
Additionally, it can be desirable to maintain the temperature in the combustion zone above about 1400.degree. F. to 1500.degree. F. to take advantage of the known silication reactions which occur above such temperatures. The silication reactions can tie up soluble compounds such as magnesium oxide, calcium oxide, and other trace metal oxides in insoluble silicate materials. These silicate materials cannot be readily leached into aquifers and, therefore, eliminating such soluble metal oxides reduces the environmental impact of in situ retorting.
Yet another reason for knowing the temperature of the combustion zone is described in U.S. Pat. No. 4,171,146 issued on Oct. 16, 1979, to Robert A. Hard. This patent described the leaching of magnesium values from the spent in situ oil shale retort and discloses that careful control of the temperature of the combustion zone in the retort is an important factor in effective recovery of magnesium values by leaching.
When the combustion zone temperature is known and it is either above or below a desired temperature, such a temperature can be adjusted to the desired temperature by varying the retort inlet composition or by varying the rate of introduction of such retort inlet mixture or by other like procedure.
Standard temperature monitoring means, such as a thermocouple or resistance temperature detector system, are not desirable for use due to the corrosive atmosphere in the in situ oil shale retort during retorting. This corrosive atmosphere at the elevated temperatures of the retorting operation can cause instrument failure and overall unreliability of such systems. In addition, such systems must be installed in the oil shale rubble after the rock has been fragmented. Thermocouples or resistance temperature detectors must be connected by wire to suitable readout instruments and such connections are too fragile to withstand the effects of the rubblization blast. Such post-rubblization installation is expensive and, in some desirable locations, may be extremely difficult or even not possible with presently existing technology.
Thus, it is desirable to provide a reliable and inexpensive method for determining the temperature of a combustion zone as it advances through an in situ oil shale retort.